The objectives of this proposal are to conduct evaluations of candidate vaccines, other biologicals, and drugs directed against certain important microbiologic agents of human infectious diseases, including viruses that infect the respiratory and gastrointestinal tracts, selected bacterial pathogens, and possibly other pathogens as requested. Evaluations will involve "phase I" and "phase II" clinical trials, to determine the safety of a preparation being studied, as assessment of relevant parameters of immunogenicity, and in the case of drugs, pharmacokinetic measurements and an assessment of effects on markers of infection or disease activity. In addition, these initial studies usually explore different dosages and routes of administration of the safety preparation. The purpose of phase II studies is to gain additional experience with a particular dosage regimen and to extend this experience into subjects of different epidemiologic or medical categories, with particular emphasis on individuals who represent the target population for the eventual utilization of the vaccine or drug. Although some assessment of the potential efficacy of the preparations may be made on the basis of data from phase I and phase II trials, determination of efficacy is made in an efficacy trial often referred to as a "late phase II" or a phase III trial. In this type of trial, the vaccine or drug is studied in large groups of subjects, who are likely to undergo naturally-acquired infections, and the effect in prevention or amelioration of such infections is compared against appropriate controls. We also propose to evaluate certain vaccines and antivirals in larger "efficacy" or field trials. We believe that the conduct of such trials is also highly relevant to the workscope of the contract and is an appropriate utilization of our resources.